531 



A. p. 



F.B. 



washed in alcohol and photographed (see Figs. 1—3) after which they 

 were stained and mounted as transparent objects and again examined. 



Description of the Specimens. 



Specimen A (see Fig. 1). This was a syncephalic monster, 

 lying transversely to the long axis of the egg, on a blastoderm limited 

 by a single sinus terminalis, and had been incubated for about 

 42 hours. The two posterior bodies, which diverged at an angle of 

 about 90° were very similar each having a well formed neural canal, 

 a notochord, and a series of mesoblastic somites numbering 16 on 

 the outside and 13 on the inside. The anterior end was quite normal 

 in appearance with a single heart into which opened the omphalo- 

 mesenteric veins, a well 

 developed fore brain a. 



with its characteristic 

 optic outgrowths and in 

 front of this an amniotic 

 fold was developing. 

 Between the single an- 

 terior and the two 

 posterior parts however 

 was a region of fusion. 

 Here the anterior three 

 somites on the inside of 

 each body have quite 

 disappeared and the 

 blastoderm itself shows 

 some sign of folding. At 

 the level of the thirteenth 



somite from the posterior end the neural canal dilates into an open groove 

 and the two inside walls quickly join thus leaving a triangular opening 

 over the region of the hind brain. In front of this, however, in the region 

 where there is an indication of the auditory pits, the two outside 

 walls unite thus forming a single internal cranial cavity. 



Specimen B. This was also a syncephalic monster, although 

 in this case the fusion had taken place at a more anterior point than 

 in the former and there is a slight malformation in the head of one 

 embryo. It was lying transversely to the long axis of the egg on a 

 blastoderm which was immediately noticeable as abnormal and had 

 been incubated for 48 hours. The two posterior parts were quite 

 complete up to the region of the mid brain, with a closed neural 



34* 



Figure 1. Photograph of Specimen A. 

 A. Amuiotic fold, A. P. Area pellucida, F.B. Fore 

 Brain, G. Open neural groove (dilated), H. Heart, 

 S. T. Sinus Terminalis. 



